Simplify, and improve the performance of, template instantiation for

declaration references. The key realization is that dependent Decls,
which actually require instantiation, can only refer to the current
instantiation or members thereof. And, since the current context
during instantiation contains all of those members of the current
instantiation, we can simply find the real instantiate that matches up
with the "current instantiation" template.

llvm-svn: 72486